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THS4567: Incorrect noise model drawing,

Michael Steffes
Guru 48155 points
Part Number: THS4567

Figure 8-1 shows the current noise in series with the inverting input - since that is a current source, it is high impedance and would open the loop. 

Normally, this is modelled as a current noise to ground at the V- node, not in series, 

Also, this equation looks wrong, is this not a differential amplifer - this is close to the single ended transimpedance noise equation - except the /2 is a mystery element. 

Also, the currents in this figure are incorrect, they do not flow out from both sides but through the diode in one direction, 

  • 0
    TI__Mastermind 23385 points

    Hello Michael,

    1. Thank you for pointing this out. It should be current source to ground at the inverting node. 
    2. From my understanding, due to the unique structure of this FDA with the integrated input common-mode control loop; the FDA as a transimpedance can be treated as two separate VFB amplifiers which would use the single-ended TIA noise equation. I agree with you where I don't see how the divided by 2 would be included in this equation since the FDA is treated as two separate VFBs rather than identical halves as suggested in the datasheet. It would be a bit more complex than suggested in the datasheet due to differential input capacitance. 
    3. True, that would also need to be corrected

      I am unsure when the next datasheet update will be for this device; but I have added it to the our literacy errors sheet. 

    Thank you,
    Sima 

  • +1 in reply to SimaJalaleddine
    Guru 48155 points

    Well, I wasn't going to regenerate the correct input referred differential noise equation, but the other thing missing is the current noise of the input Icm term as that is unique to this solution. 

    Here is the added noise current term plot - incidentally if I do a root (2q100uA) at the far right of this plot I get a 5.7pA term, not the 3pA shown, and on each side